1. Field of the Invention
The invention relates to a novel phosphorus-containing polymer compound, a method for synthesizing the phosphorus-containing polymer compound, an antioxidant including the phosphorus-containing polymer compound, a high-durability polymer electrolyte composite including the phosphorus-containing polymer compound and a fluoropolymer electrolyte, an electrode and a high-durability fuel cell.
2. Description of the Related Art
Various types of antioxidants to be added to various sorts of organic materials have been developed. For example, a phenol compound, an amine compound, a sulfur compound, and a compound containing phosphorus are known. Also, not only low molecular weight antioxidants but also high molecular weight antioxidants have been developed. However, in order to apply an antioxidant to a polymer electrolyte or the like for a fuel cell, it is desired that a polymer antioxidant be developed whose outflow to the outside of a system of the fuel cell is small even in a humid condition, which has higher heat resistance, and which can maintain performance of the fuel cell for a long time.
A polymer electrolyte is a solid polymer material having an electrolytic functional group such as a sulfonic acid group in a polymer chain thereof. The polymer electrolyte is strongly bonded to a certain ion, and is selectively pervious to a positive ion or a negative ion. Therefore, the polymer electrolyte is formed as particles, fiber or membranes, and is applied to various uses such as electrodialysis, diffusion dialysis and battery diaphragms.
For example, a reformed gas fuel cell has a proton conductive polymer electrolyte membrane with one electrode provided on each surface thereof. In the reformed gas fuel cell, hydrogen gas, which is obtained by forming a low molecular weight hydrocarbon such as methane and methanol, is supplied to one of the electrodes (fuel electrode) as fuel gas, while oxidizing gas or air is supplied to the other electrode (air electrode) as an oxidant, such that an electromotive force is generated. Water electrolysis is a method for generating hydrogen and oxygen by electrolyzing water using a polymer electrolyte membrane.
In the case of a fuel cell or water electrolysis, a peroxide is generated in a catalyst layer formed on an interface between the polymer electrolyte membrane and the electrode, and the generated peroxide becomes a peroxide radical while being diffused, and causes a reaction which deteriorates the polymer electrolyte membrane. Therefore, it is difficult to use a hydrocarbon polymer electrolyte membrane having low oxidation resistance. Accordingly, in the fuel cell and water electrolysis, a perfluoro sulfonic acid membrane having high proton conductivity, and high oxidation resistance is generally used.
Salt electrolysis is a method for generating sodium hydroxide, chlorine, and hydrogen by electrolyzing a sodium chloride solution using a polymer electrolyte membrane. In this case, the polymer electrolyte membrane is exposed to chloride, and a high-temperature solution containing a high concentration of sodium hydroxide. Therefore, it is impossible to use a hydrocarbon polymer electrolyte membrane having low resistance to chloride, and the high-temperature and concentrated sodium hydroxide solution. Accordingly, a polymer electrolyte membrane for salt electrolysis, e.g., a perfluoro sulfonic acid membrane, is generally used which is resistant to chlorine and a high-temperature and concentrated sodium hydroxide solution. A carboxylic acid group may be partially introduced to a surface of the perfluoro sulfonic acid membrane in order to prevent back-diffusion of generated ions.
A fluoropolymer electrolyte membrane typified by the perfluoro sulfonic acid membrane has C—F bonding and therefore has a considerably high chemical stability. Accordingly, the fluoropolymer electrolyte is used not only as a polymer electrolyte membrane for a fuel cell, water electrolysis or salt electrolysis, but also as a polymer electrolyte membrane for hydrohalic acid electrolysis. Also, the fluoropolymer electrolyte is widely applied to a humid sensor, a gas sensor, an oxygen concentrator, and the like, due to proton conductivity thereof.
Particularly, a membrane formed of the fluoropolymer electrolyte typified by the perfluoro sulfonic acid membrane which is known under a trade name of Nafion (manufactured by Du Pont Co., Ltd.), is used as an electrolyte membrane to be used under a severe condition, due to considerably high chemical stability thereof.
However, the fluoropolymer electrolyte has shortcomings that it is difficult to produce, and is extremely costly. Therefore, it has been considered that oxidation resistance be enhanced by quenching a hydrogen peroxide radical generated in a system of the hydrocarbon polymer electrolyte membrane.
Compared with the fluoropolymer electrolyte typified by Nafion®, the hydrocarbon polymer electrolyte membrane has advantages that it is easy to produce and is low in cost. However, there remains a problem concerning the hydrocarbon polymer electrolyte membrane that oxidation resistance thereof is low, as mentioned above. The low oxidation resistance is due to the facts that a hydrocarbon compound generally has low resistance to a radical, and that an electrolyte having a hydrocarbon skeleton is likely to cause deterioration reaction due to the radical (an oxidation reaction due to a peroxide radical).
Therefore, as a high-durability polymer electrolyte, and a high-durability polymer electrolyte composite, which have oxidation resistance that is equal to or higher than that of a fluoropolymer electrolyte or that is sufficient for actual use, and which can be produced at a low cost, Japanese Patent Laid-Open Publication No. 2000-11755 discloses a high-durability polymer electrolyte which is formed of a polymer compound having a hydrocarbon portion and to which a functional group containing phosphorus is introduced, and Japanese Patent Laid-Open Publication No. 2000-11756 discloses a high-durability polymer electrolyte composite which is obtained by mixing a polymer compound having an electrolytic functional group and a hydrocarbon portion with a compound containing phosphorus.
However, in the case where the hydrocarbon polymer electrolyte membrane is used in the fuel cell, since the antioxidant electrolytes disclosed in Japanese Patent Laid-Open Publication No. 2000-11755 and Japanese Patent Laid-Open Publication No. 2000-11756 have high gas impermeability, each of the antioxidant electrolytes greatly hinders fuel gas (hydrogen or the like) or oxidizing gas (oxygen, air, or the like) from contacting a catalyst (platinum or the like), and considerably deteriorates performance of the fuel cell, when being provided in the electrode. As described so far, there exists a problem in the combination of the hydrocarbon polymer electrolyte, and the functional group containing phosphorus or the compound containing phosphorus.